Some position determining systems determine certain receiver variables, such as location, velocity and certain parameters of the receiver's clock, by receiving and processing a signal containing location-determining signals transmitted by a plurality of sources. These receiver variables are estimated by first estimating certain signal variables, such as code-phase and Doppler frequency and then by performing a triangulation operation that is based on an equation relating the signal's code-phase to the signal's propagation delay, and on the linear relationship between the propagation delay and the geometric distance from the source to the receiver. The latter relationship assumes that the transmitted signal reaches the receiver by traveling on a straight line and that the speed of light is constant along this path. In certain environments, “multipath” phenomena such as reflection, diffraction, diffusion, scattering, may in-validate these assumptions by generating multiple copies of the signal that reach the receiver with longer delays. As a consequence, the correlation function used to the detect the signal may contain several large peaks. Furthermore, the direct-path peak may be significantly attenuated or eliminated due to occlusion or negative interference, and the highest detected peak may correspond to a signal received through a reflected path.
The straightforward least-square triangulation methods that assume straight unaltered propagation paths may perform very poorly in multipath environment. A method to mitigate multipath effects is disclosed in U.S. Pat. No. 5,630,208, “Adaptive Multipath Equalization” by Enge et al. This method is based on the identification of certain features of the correlation function near the peak. In one example, the correlation is sampled at two pairs of points symmetrically located on each side of the peak. One pair of points are close to each other (narrow spacing), the other pair are further apart (wide spacing). The correlation values at these four points are combined to mitigate multipath effects. In a second example, multipath signal strengths and phases are estimated by using multiple samples of the correlation function. This second approach assumes that path delays of the direct signal and of the multipath signals can be determined separately. Three approaches are proposed to determine path delays: (1) identification of slope transition points in the correlation function; (2) cepstrum processing of the received signal, using Fourier transform analysis; and (3) use of a grid of time points on the correlation function domain, and identification of time values, associated with certain solution parameters of the least mean squares analysis that have the largest absolute values, as times of arrival of the direct and multipath signals.
These methods perform well only when the distorted correlation function contains the direct-path peak and only a small additional number of distinct and delayed peaks. Therefore, there is a need for a method to mitigate multipath phenomena that overcomes these limitation.